Crossing Page Boundaries

As described in Section 2, programs are held in either ROM or program RAM, both of which are divided into pages.
Each page consists of 256 8 -bit locations.
Addresses 0 through 255 comprise the first page,256-511 comprise the second page, and so on.

In general, it is good programming practice to never allow program flow to cross a page boundary except by using a JUN or JMS instruction.

The following example will show why this is true. Suppose a program in memory appears as below:

If the accumulator is non-zero when the JCN is executed, program control will be transferred to location 200, as the programmer intended.
Suppose now that an error discovered in the program requires that a new instruction be inserted somewhere between locations 200 and 253.
The program would now appear as follows:

Since the JCN is now located in the last two locations of a page, it functions differently.
Now if the accumulator is non-zero when the JCN is executed, program control will be erroneously transferred to location 456, causing invalid results. Since both the JUN and JMS instructions use 12-bit addresses to directly address locations on any page of memory, only these instructions should be used to cross page boundaries.